The manufacture of compression molded composites from a mixture of wood chips or fibers and a polyisocyanate binder is well known. The isocyanate binders are excellent adhesives for the wood fibers, but they also promote unwanted adhesion of the composite boards to the metal press platens. Efforts to develop a release agent composition that will provide multiple press cycles have consumed decades of research, with only modest success. For any release agent to be effective it must provide at least a limited capability for multiple press cycle operation. This is because, at minimum, there are mistakes and interruptions in the spraying of external mold release agent between pressing cycles that could result in catastrophic press sticking, if there were not at least some effective amount of mold release residue on the platens from a previous molding cycle. This is called “release memory”. In addition to this minimum requirement, there is a strong economic driving force for the reduction or elimination of the need for mold release treatment of the press platens between pressing cycles. Clearly, the more cycles that can be achieved between mold release applications, the faster and more economical the overall process will be.
Conventional release agents long used in the polyurethane industry, such as fatty acids and salts thereof, do not provide satisfactory release memory or the capability of running multiple press cycles between release treatments.
Specialized external release agents and internal/external combinations have been reported that can provide release memory and multiple cycling capability. Some of these require elaborate and costly processes for pre-treatment of the press platens, such as multiple coating and baking the platens. Still other release agent systems that can provide multiple release capability and release memory must still be applied with sufficient frequency that they begin to build up on the platens. Release agents based on waxes or silicones exhibit this problem. The buildup must be removed periodically, or it will cause surface defects on the pressed boards. The removal of press buildup involves a complex and expensive cleaning process, which interrupts production. Some examples of known release agent compositions include those disclosed in U.S. Pat. No. 4,110,397; U.S. Pat. No. 4,742,144; U.S. Pat. No. 4,740,575; U.S. Pat. No. 4,528,154; U.S. Pat. No. 4,772,442; U.S. Pat. No. 4,933,232; U.S. Pat. No. 4,539,166; U.S. Pat. No. 4,024,090; USH 000444; EP 269869; WO 98/00464; and JP 49023259. Despite all of this known release technology, which has been available for years, a satisfactory solution to the release problem for fully isocyanate bonded compressed wood composites (such as oriented strand board, OSB) is still not in hand.
In applications where isocyanate binders are currently used (including OSB) it is still necessary to apply a surface layer of a non-isocyanate bonded composite wood material, interposed between the platen surface and the isocyanate-bonded “core” of the board, in order to prevent unwanted adhesion. This complicated surface-core sandwiching process has increased the cost of board manufacturing, and has prevented the industry from realizing the full benefits of isocyanate binders. The known advantages of using isocyanate adhesives include improved board physical properties.
Clearly there is a need for an improved release agent composition that can provide multiple release capability (release memory) using a simple application process, and that does not cause excessive press buildup. The present invention provides such an improved release agent composition, and an improved process for making isocyanate bonded lignocellulosic composites that can use the isocyanate binder in both the core and the surface layers of the composite board.